Thick polymer coating of a substrate apparatus and method

ABSTRACT

A method for applying a polymer coating to a substrate wherein the resultant layer of polymer on the substrate has a substantial thickness. A mixture of polymer material, including reactor bead polymer and ground polymer, may be used in a powder coating process to achieve thicker polymer layers.

RELATED APPLICATIONS

This utility patent application claims the benefit of U.S. ProvisionalPatent Application Ser. No. 62/554,467, filed on Sep. 5, 2017, which ishereby incorporated by reference in its entirety.

BACKGROUND The Field of the Invention

The invention relates to formulations, apparatus and methods used tocoat surfaces with a polymer, and more specifically to polymer coatingof substrates using a polymer mixture and method that enables increasedthickness of polymer coatings in a more efficient manner.

Background

Polymeric film-forming materials may be applied as coatings forfunctional, protective purposes and even for decoration. A variety ofcoating methods are available. Most methods may be used with a widerange of base materials and coating compositions.

A variety of methods employing powdered plastics and resins may be usedcommercially to apply polymeric coatings to various articles orsubstrates. Such methods may include fluidized bed, plasma spray,electrostatic spray, electrostatic fluidized bed, and hot flocking, aswell as combinations and variations of these methods.

In one example using an electrostatic spray process, the coating powderis withdrawn from a reservoir in an air stream and electrostaticallycharged in the high voltage corona field of a spray gun. The chargedpowder particles are attracted to the grounded metal object, orsubstrate, to be coated and adhere to it by electrostatic attraction.The coated substrate may then be placed in an oven and the coating isfused to form a continuous film. If the powder is sprayed on a preheatedarticle or substrate, the powder melts and fuses directly on the hotsurface. Further heating to fuse or cure the coating may be requireddepending on the type of coating powder and the substrate. Essentiallythe same process may be repeated to obtain a thicker layer of polymer ona substrate.

In another example using a plasma-coating method, a high temperatureplasma is established in an inert gas such as nitrogen, and the coatingpowder is introduced at the periphery of the plasma. The particles meltand are propelled at high velocity to the substrate where they form afilm.

In another example using a hot-flocking technique, powders may bedispersed in air and sprayed or blown onto the preheated article orsubstrate, where the powder melts and forms a coating. In a variation ofthis technique, small parts are preheated and dropped into a bed ofpowder kept in a mobile state by vibration and the parts are completelycoated with an unfused layer of powder on the surface.

All powder-coating methods may generally be referred to asfusion-coating methods or processes, since the material must be meltedand fused to form a continuous coating at some stage in the process.

Generally, a powdered or granulated polymer is used to electrostaticallypowder coat a substrate. It is recommended that a ground, powdered, orgranulated polymer be used for electrostatic powder coating. Put anotherway, it is not recommended to use unground, rough, or reactor beads ofthe polymer when electrostatically powder coating a substrate.

The average thickness of a polymer layer was about 1 mm, and it wouldtake many cycles to build up a significant polymer layer on a substrate.Some methods were developed that could result in a polymer layer ofabout 2-3 mm of thickness, and there can be referred to as “high build”processes. It would be an advance in the art to have a method thatutilizes a cheaper polymer material and can obtain a greater polymerthickness, require fewer cycles in the oven, and still maintain andimprove the physical and chemical properties of the coating.

BRIEF SUMMARY OF THE INVENTION

The present invention or method described herein may include a processfor coating a substrate using a mixture of reactor beads of polymermixed with the usual ground or powdered polymer using an electrostaticpowder coating process.

Then a specific temperature range is used to promote melting of thepolymer(s) without significant run-off of the polymer coating.

In one embodiment, a method for coating a substrate with a polymer maycomprise selecting a reactor bead polymer, selecting a ground polymer,mixing a suitable amount of the reactor bead polymer and a suitableamount of the ground polymer into a polymer mixture, providing asubstrate, heating the substrate, coating the substrate with the polymermixture, and heating the substrate and the polymer mixture. The steps ofcoating the substrate with another layer of the polymer mixture andheating the substrate and the additional layer of the polymer mixturemay be repeated until a polymer thickness of at least 6.0 mm, andgenerally between 7.5 mm and 10 mm, is achieved on the substrate.

An embodiment of the method may include repeating the steps of coatingthe substrate with another layer of the polymer mixture and heating thesubstrate and the additional layer of the polymer mixture less than 15times to achieve the polymer thickness. The polymer mixture may beapproximately 50% the reactor bead polymer and approximately 50% theground polymer. The polymer mixture may include at least one additive,such as graphene, nanofibers, nanotubes, or the like. The coating may beachieved by a powder coating process.

Generally, the substrate and the polymer mixture are heated until thepolymer mixture is substantially melted. The heating may be done at atemperature of at least 260° C. Any temperature suitable for melting thepolymer mixture and not harming the substrate may be used. The substratemay be of any suitable composition and may include a compositesubstrate.

In one embodiment, a polymer mixture may include a reactor bead polymerand a ground polymer and an additive, such as graphene. A substrate maybe provided and the substrate may be prepared for the coating process ina manner that promotes a more even and complete resultant layer ofpolymer.

The substrate may be heated and then cooled. A primer layer may beapplied to the substrate before any polymer mixture is coated onto thesubstrate. The substrate may be coated with the polymer mixture toprovide a first layer of polymer. The substrate and first layer ofpolymer may be heated, but the first layer of polymer may not completelymelt, resulting in the first layer of polymer having bumps. The polymermixture may be applied to provide a second layer of polymer over thisun-melted first layer of polymer. Then, the layers of polymer and thesubstrate may be heated again. This process may result in both the firstlayer of polymer and the second layer of polymer completely melting,resulting in a smooth finished layer of polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the present invention will become more fullyapparent from the following description and appended claims, taken inconjunction with the accompanying drawings. Understanding that thesedrawings depict only typical embodiments of the invention and are,therefore, not to be considered limiting of its scope, the inventionwill be described with additional specificity and detail through the useof the accompanying drawings in which:

FIG. 1 is a schematic block diagram of a method described herein; and

FIG. 2 is a schematic block diagram of a method described herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It will be readily understood that the components of the presentinvention, as generally described and illustrated in the drawingsherein, could be arranged and designed in a wide variety of differentconfigurations. Thus, the following more detailed description of theembodiments of the system and method of the present invention, asrepresented in the drawings, is not intended to limit the scope of theinvention, as claimed, but is merely representative of variousembodiments of the invention. The illustrated embodiments of theinvention will be best understood by reference to the drawings, whereinlike parts are designated by like numerals throughout.

In one embodiment, a method for achieving very high builds, or thicklayers, of PVDF (i.e., KYNAR®) fluorocarbon electrostatic powdercoatings is disclosed. Any suitable PVDF powder may be used, includingwithout limitation, SOLER® KF and other similar compounds. The principleobserved may apply to other partially fluorinated polymers such as ETFEand ECTFE (i.e., TEFZEL® and HALAR®, respectively). Any suitablepartially fluorinated polymers may be used, including withoutlimitation, FLUON® and other similar compounds.

KYNAR® PVDF resins have a high coefficient of thermal expansion. Thishas been a difficulty and a challenge in the powder coating of vessels,pipes, tanks, agitators, mixers, pump housings, valves, and othersubstrates.

The process of powder coating may place a properly prepared metal orother substrate in an oven and heated to about 500° F. (about 260° C.).After the part or substrate to be coated reaches equilibrium throughout,it may be removed from the oven and a primer layer may be applied. Thepurpose of the primer layer is to aid in adhesion and reduce thelikelihood of bubbling, blisters, and peeling over time. Following theapplication of the primer layer, successive cycles of heating and powderapplication may follow. Generally, 8-12 mils (“mils” referring to athousandth of an inch), or about 0.2032 mm-0.3048 mm, of powder areapplied in one of these cycles. The desired minimum thickness is usuallygreater than 40 mils (or 1 mm).

As the thickness of the coating increases, especially with KYNAR® PVDF,the stresses upon the adhesive bond to the substrate increase. Failureshave been observed where the stress has become too great and overcomesthe adhesion of the coating to the substrate, thereby creating voids.

In order to compensate and to reduce these stresses, additional KYNAR®copolymer resin mixtures may be dry blended into the standard KYNAR®2850 PC resin. These other KYNAR® resins are collectively known as KYNARFLEX® or SUPER FLEX®. They contain a comonomer of HFP,hexafluoropropylene. The crystalline structure of the PVDF is reducedand the tensile modulus drops as well. By utilizing these more flexiblegrades the resultant stresses on coatings can be reduced significantly.

In one embodiment, KYNAR SUPERFLEX® 2501 copolymer may be utilized.Currently, this version of this material has a relatively high contentof HFP. The price of this material is significantly lower than otherversions of this or similar material. The reason for the lower price isbecause it is reactor bead and has not gone through any subsequentprocessing steps, such as pelletizing, extruding or grinding intopowder. The reactor bead form is believed to be unsatisfactory forcoating, as it does not flow like a ground KYNAR® powder.

However, mixing the reactor bead with the standard 2850 PC KYNAR® inratios of 10-90, 20-80, 30-70, and 50-50 may be superior in severalways. For example, the likelihood of disbanding is reduced. Also,smoother coatings may be achieved as compared to the standardformulation of 2850 PC. Also, thicker coatings may be achieved. Thus, itis an unexpected benefit to include the reactor bead KYNAR® as part ofthe polymer mixture used for powder coating.

In one embodiment, a polymer mixture, or mixture, of 50-50 KYNAR® 2501and 2850 may be utilized while attempting to apply a very thick KYNAR®coating on various substrates. Just below the melt temperatures of theKYNAR® powder mixture, bumps may form in the coating on metalsubstrates. In subsequent cycles in and out of the oven, these bumps maynot melt and flow.

More powder may be added to cover the bumps, which may be consideredreminiscent of covering the coating with something that looked like“powdered sugar” over the whole surface covering the bumps protrudingfrom the surface of the substrate. Then, returning the parts, orsubstrate, to the oven for flow out may result in or allow this“powdered sugar” layer to completely fuse and/or cover the bumps aftersufficient time in the oven. This process may be repeated on multipleparts or substrates. This process may be repeated for one or more cycleswhile polymer coating a part or substrate.

It may be that these bumps are holding an excess of powder on the metalsubstrate, much in the way of a composite giving form and reinforcementto additional powder until the melt occurs. Generally, reactor beadpolymers are not considered suitable for coating because the reactorbead does not flow like ground KYNAR® powder for coating. The lack offlow may explain why the bumps form slightly below the melt transitiontemperature. In any case, it may be possible to perform spray coatingsin each cycle having from 10-15 mils (or about 0.254 mm to 0.381 mm) to30-40 mils (or about 0.762 mm to 1.0 mm) per cycle.

A method may comprise selecting an appropriate polymer, mixing a portionor reactor bead polymer with ground polymer, placing a properly preparedmetal or other substrate in an oven and heating the substrate to about500° F. (about 260° C.) and letting the substrate reach equilibrium,remove from the oven, applying a primer coat, heating the substrate andprimer coat, applying the polymer mixture by powder coating withapproximately 8-12 mils (about 0.2032 mm-0.3048 mm) layer of polymerpowder in each cycle, and heating the substrate and polymer mixture tomelt the polymer. A “cycle” may be considered the steps of applying alayer of polymer powder coating and then heating the substrate andpolymer to about 500° F. (about 260° C.) and letting the substrate reachequilibrium to achieve the proper melt or flow of the polymer.Successive cycles may be utilized to achieve a desired minimum thicknessof polymer coating. The desired minimum thickness is usually greaterthan 40 mils (or 1.0 mm).

Referring more particularly to FIG. 1, a method 10 may comprise a numberof steps to provide a thick polymer coating to a substrate. For example,and not by way of limitation, a method 10 may comprise selecting apolymer mixture 20.

A suitable polymer mixture may include almost any ratio of a suitablepolyvinylidene fluoride (PVDF) polymer, wherein the ratio is comprisedof a reactor bead form of the PVDF polymer and a ground or pelletizedform of the PVDF polymer. The ratio of reactor bead polymer to groundpolymer may be anywhere from 10-90 to 50-50. KYNAR® may be considered asuitable polymer, as well as any other polymer similar to KYNAR® or anyother polymer that may be used or desired as a polymer coating onsubstrates. A suitable polymer mixture may be comprised of oneparticular polymer and/or one or more forms of that polymer.

A suitable polymer mixture may also include an additive, or filler. Forexample and not by way of limitation, a suitable additive may includegraphene, nanofibers, nanotubes, silica, mica and/or any similarmaterial. For example and not by way of limitation, a suitable additivemay include inorganics, such as manganese disulphide, aluminum oxide,tungsten carbide and/or any similar material. The amount of additive orfiller utilized in a polymer mixture may be from about 0.1% to 10% byweight depending on the filler material selected for use.

The inclusion of an additive, or filler, in a polymer mixture may bedescribed as utilizing a “tortuous path” method. The spaces between themolecular chains may be filled and barriers created from flakes of asuitable additive.

A method 10 may comprise selecting a substrate 30 to be coated with theselected polymer mixture. Virtually any substrate may be selected.Generally, the substrate may be a metal substrate. A substrate may alsobe a composite substrate, for example and not by way of limitation, asubstrate that may be a composite of metal and/or fiberglass.

A substrate may be considered suitable as long as it can withstand theheating process required to substantially melt the polymer being appliedto the substrate. In other words, any substrate may be used in theprocess described herein as long as the substrate has a higher meltingpoint than the polymer coating to be applied to the substrate.

A substrate may require a certain amount of processing before thesubstrate is heated. For example, the substrate may be cleaned, cornerson the substrate may need to be rounded, or unwanted abnormalities ordefects on the surface of the substrate may need to be addressed.Moreover, the surface of the substrate may be grit blasted inpreparation for the coating process.

A method 10 may comprise heating the substrate 40. A substrate may beheated to about 500° F. (about 260° C.), or about 550° F. (about 288°C.). The substrate may then be allowed to reach equilibrium. Thesubstrate may also be removed from the oven to allow for cooling.

A method 10 may comprise applying a primer coat to the substrate 50. Aprimer coat may be of any suitable material. For example and not by wayof limitation, a primer may be a KYNAR® expoxide primer. Such a primermay be a mix of thermosetting and thermoplastic resins.

A method 10 may comprise heating the substrate and primer 60. Thesubstrate and primer may be heated to melt the primer coat onto thesubstrate in preparation for the polymer mixture coating. The primer maybe used to help adherence of the polymer mixture to the substrate and toprevent peeling or bubbles.

A method 10 may comprise applying the polymer mixture 70. Applying thepolymer mixture may be accomplished by any suitable means, includingwithout limitation, powder coating and hot-flocking.

A method 10 may comprise heating the substrate and polymer mixture 80.The substrate and polymer mixture may be heated to about 500° F. (about260° C.). The substrate and polymer mixture may be heated to about 440°F. (about 227° C.), and maybe as low as 400° F. (about 205° C.). Incertain embodiments, the temperature used for heating a substrate and apolymer mixture may be as low as 75° F. (about 24° C.).

The heating is performed to allow the polymer mixture to melt or flowonto the substrate. Thus, the heating may be performed at anytemperature that allows for or produces the desired melting or flowingof the polymer mixture onto the substrate.

A method 10 may comprise achieving a minimum desired polymer thickness90. Generally, multiple “cycles” may be required to achieve the minimumdesired polymer thickness. A “cycle” may be defined as the steps ofapplying the polymer mixture to the substrate and heating the substrateand polymer mixture to melt or flow the polymer onto the substrate.

In one embodiment as described herein, the step of applying the polymermixture may result in a layer of polymer mixture that is about 10-15mils (or about 0.254 mm to 0.381 mm) to 30-40 mils (or about 0.762 mm to1.0 mm) thick per cycle. This significantly decreases the processingtime for polymer coating a substrate, but allows for significantlyincreased thicknesses of coatings.

Referring more particularly to FIG. 2, another embodiment of a method 10may be described. For example, and not by way of limitation, a method 10may comprise selecting a KYNAR® polymer mixture 120. A KYNAR® polymermixture may comprise a 50-50 blend of KYNAR® 2501 reactor bead andKYNAR® 2850 PC, which KYNAR® 2850 PC is generally considered a standardpowder for powder coating applications. The mixture of KYNAR® 2501 withthe KYNAR® 2850 may reduce the amount of shrinkage and pull away fromthe substrate. Also, the KYNAR® 2501 may be considered valuable ordesirable in coating application because the modulus is lower and theflexibility is higher due to the higher concentration ofhexafluoropropyl (HFP) comonomer added to the KYNAR® 2501.

A method 10 may comprise selecting a metal substrate 130. A metalsubstrate to be coated may be cleaned and prepared for coating. Forexample, and not by way of limitation, sharp edges may be radiused andany weld splatter removed as necessary. The surfaces of the substrate tobe coated may be grit blasted to a NACE number 1 or 2 grade profile. Thesurfaces of the substrate that are not coated may be either masked orcovered with a mold release agent. The surfaces of the substrate may bedusted prior to the coating process.

A method 10 may comprise heating the metal substrate 140. For exampleand not by way of limitation, the metal substrate may be hung in theoven and the temperatures of the oven and substrates reach anequilibrium of about 550° F. (or about 288° C.). It is not unusual thatthe parts or substrates to be coated may be about 20° F. to 30° F.(about 5° C. to 10° C.) lower than the thermostat setting of the oven.In order to insure uniform heating, and also to prevent drips and theruns, the parts or substrates may be rotated about their major axisconstantly, in and out of the oven. Temperatures may be monitored with aremote infrared sensing thermocouple.

The process of heating certain parts or substrates may take a minimum oftwo (2) hours based on the thickness of the metal substrate. Setting theoven temperature to about 550° F. (or about 288° C.) also assures thatany organic compounds are flashed off or turned to carbon dust. Asubstrate may be rotated about a major axis at approximately 10-12 RPM,in and out of the oven. When the parts or substrate are at or near about550° F. (or about 288° C.) they may be removed from the oven.

A method 10 may comprise applying an expoxide primer coat 150. Anexpoxide primer coat may be of any suitable composition. The thicknessof a primer layer may be approximately 8-12 mils (about 0.2032 mm-0.3048mm). In certain embodiments, when the parts or substrates are relativelythick, a layer of top coat may be applied to the primered surface.

A method 10 may comprise returning the metal substrate to the oven andheating the metal substrate and expoxide primer 160. The metal substrateand expoxide primer coating, with or without top coat, may be reheatedabove the melt temperature of the KYNAR®. The metal substrate may bekept in the oven at about 380° F. (193° C.) for approximatelythirty-five (35) minutes. The time of the reheating may depend on howlong it takes for the primer, with or without top coat, to properly meltand fuse.

A method 10 may comprise applying the KYNAR® polymer mixture 170. AKYNAR® polymer mixture, or other suitable polymer, may be applied to thesubstrate, which generally has a primer coat. Generally, anotherapproximately 8-12 mils (about 0.2032 mm-0.3048 mm) of polymer mixturecoating may be applied before it cools too much, which would increasethe risk of powder drop off.

The formation of bumps or vertical projections from the coated substratesurface may occur at approximately the melt temperature of the KYNAR®2850, between approximately 380° F. and 400° F. (or approximately 193°C. and 205° C.). Generally, the coating process is stopped and the partsor substrates are put back in the oven before too many bumps appear.

A method 10 may comprise heating the metal substrate and KYNAR® polymermixture 180. This heating step may be performed at any suitabletemperature and for any suitable length of time. For example and not byway of limitation, in the oven may be set at about 380° F.-400° F. (orapproximately 193° C.-205° C.) for approximately thirty-five (35)minutes. The heating step is intended to properly melt and/or fuse theKYNAR® polymer mixture.

A method 10 may comprise achieving a minimum desired KYNAR® polymerthickness 190. The minimum desired KYNAR® polymer thickness may be anysuitable or desired thickness. Generally, when the method disclosedherein is used, the resulting KYNAR® polymer thickness may beapproximately 300 mils or more (or over 7.6 mm).

In certain embodiments of a method, bumps or protrusions may appear onthe surface of the substrate, even after heating. A method may includingcoating or flooding the substrate surface with powder even though nomelt is occurring, or the melt is incomplete. The substrate may then beheated at approximately 400° F. (approximately 205° C.) and forapproximately forty (40) minutes to encourage any bumps to melt or flowout.

In certain embodiments of a method, the steps of coating a substratesurface with the KYNAR® polymer mixture, with or without bumps on thesurface of the substrate, may be repeated as required to achieve theminimum desired thickness. The use of the KYNAR® polymer mixture, or anysuitable polymer mixture that includes reactor bead and ground polymer,may be used to enable thicker coatings of polymer per cycle.

After the desired minimum thickness is achieved, the parts or substratesmay be allowed to cool in any suitable manner. For example and not byway of limitation, the substrate may be allowed to hang in the oven andcontinue to be rotated as the door of the oven is left slightly open andthe temperature setting of the oven is at zero. This slow cooling androtation may last for approximately three (3) hours.

Any suitable finish work may also be performed on the substrate or part,as may be desired to prepare a part for its particular placement anduse.

What is claimed and desired to be secured by United States LettersPatent is:
 1. A method for coating a substrate with a polymer, themethod comprising: selecting a reactor bead polymer; selecting a groundpolymer; mixing a suitable amount of the reactor bead polymer and asuitable amount of the ground polymer into a polymer mixture; providinga substrate; heating the substrate; coating the substrate with thepolymer mixture; and heating the substrate and the polymer mixture. 2.The method of claim 1, further comprising: coating the substrate withanother layer of the polymer mixture; heating the substrate and theadditional layer of the polymer mixture; and repeating these steps untila polymer thickness of between 7.5 mm and 10 mm is achieved on thesubstrate.
 3. The method of claim 2, wherein the steps of coating thesubstrate with another layer of the polymer mixture and heating thesubstrate and the additional layer of the polymer mixture are repeatedless than 15 times to achieve the polymer thickness.
 4. The method ofclaim 1, wherein the polymer mixture is approximately 50% the reactorbead polymer and approximately 50% the ground polymer.
 5. The method ofclaim 2, wherein the coating is achieved by a powder coating process. 6.The method of claim 2, wherein the substrate and the polymer mixture areheated until the polymer mixture is substantially melted.
 7. The methodof claim 2, wherein the heating is done at a temperature of at least260° C. and the substrate is rotated during heating.
 8. The method ofclaim 1, wherein the substrate is a composite substrate.
 9. The methodof claim 1, wherein the polymer mixture further comprises at least oneadditive selected from the group consisting of graphene, nanofibers, andnanotubes.
 10. A method for coating a substrate with a polymer, themethod comprising: selecting a reactor bead polymer; selecting a groundpolymer; mixing the reactor bead polymer and the ground polymer into apolymer mixture; providing a substrate; heating the substrate; coatingthe substrate with a primer; coating the substrate with the polymermixture to provide a first layer of polymer; heating the substrate andthe polymer mixture, wherein the first layer of polymer is notcompletely melted; coating the first layer of polymer with the polymermixture to provide a second layer of polymer; and heating the substrate,the first polymer layer and the second polymer layer until the first andsecond polymer layers are completely melted.
 11. The method of claim 10,wherein the polymer mixture has a ratio of approximately 1:1 of reactorbead polymer to ground polymer.
 12. The method of claim 11, wherein thepolymer mixture further comprises at least one additive selected fromthe group consisting of graphene, nanofibers, and nanotubes.
 13. Themethod of claim 12, further comprising: coating the substrate and thecompletely melted first and second polymer layers with the polymermixture to provide an additional layer of polymer; heating the substrateand the additional layer of polymer; and repeating these steps until apolymer thickness of at least 7.5 mm is achieved on the substrate. 14.The method of claim 10, wherein the polymer mixture further comprises atleast one additive selected from the group consisting of graphene,nanofibers, and nanotubes.
 15. The method of claim 14, furthercomprising: coating the substrate and the completely melted first andsecond polymer layers with the polymer mixture to provide an additionallayer of polymer; heating the substrate and the additional layer ofpolymer; and repeating these steps until a polymer thickness of at least7.5 mm is achieved on the substrate.
 16. The method of claim 10, whereinthe substrate is a composite substrate.
 17. A method for coating asubstrate with a polymer, the method comprising: selecting a reactorbead polymer; selecting a ground polymer; mixing the reactor beadpolymer and the ground polymer into a polymer mixture that has a ratioof approximately 1:1 for reactor bead polymer to ground polymer;providing a substrate; heating the substrate to approximately 550° F.;allowing the substrate to cool for approximately 30 minutes; coating thesubstrate with the polymer mixture; heating the substrate and thepolymer mixture at approximately 440° F. for approximately 45-60 minutesand rotating the substrate during heating; and allowing the substrateand the polymer mixture to cool for approximately 30 minutes.
 18. Themethod of claim 17, further comprising: coating the substrate withanother layer of the polymer mixture; heating the substrate and theadditional layer of the polymer mixture at approximately 440° F. forapproximately 45-60 minutes; allowing the substrate and the additionallayer of the polymer mixture to cool for approximately 30 minutes; andrepeating these steps until a polymer thickness of at least 8.0 mm isachieved on the substrate.
 19. The method of claim 19, wherein thecoating the substrate with another layer of the polymer mixture mayoccur when a previous layer of polymer mixture has not completely meltedafter the heating.
 20. The method of claim 17, wherein the polymermixture further comprises at least one additive selected from the groupconsisting of graphene, nanofibers, and nanotubes.